This invention relates to coating of water-containing plastic sheet and production of oriented polymer film therefrom. In particular, it relates to methods for applying aqueous polymer coating compositions to water-containing substrates during film manufacture.
A variety of polymers are suitable for making films or foil products coated with water-dispersible, film-forming resins, such as vinylidene chloride polymers (saran-type), epoxy resins and the like. Film products are often manufactured by casting or extruding a continuous web, tubular shape or other sheet-like material, which is subsequently stretched to provide molecular orientation and treated to remove volatile components. Often such films require an additional layer of a different polymer to achieve the desired sealing or bonding properties, gas barrier characteristics, etc. Conventional systems often apply another coating to the film substrate as an aqueous dispersion.
Various films, such as polyolefins, polyesters, polyamides, acrylics or many others are treated by coating with various resin coatings. Acrylonitrile polymers and other resinous materials may be treated with saran dispersions to obtain the desired product.
In U.S. Pat. No. 4,197,356 a cast oriented polyacrylonitrile (PAN) is treated with an aqueous emulsion containing saran polymer. The film can be manufactured by solvent casting with subsequent removal of the solvent. After orientation and drying, the PAN film is treated with an aqueous vinylidene chloride copolymer dispersion and redried to obtain a coated finished product. Saran-type coatings are applied to a variety of substrate materials in U.S. Pat. Nos. 3,262,808, 3,271,178, 3,506,751 and 3,988,157, as subbing layers, heat sealing promoters, barrier layers and photographic film support, for instance.
Prior art film manufacturing techniques usually treat the formation, orientation and coating treatment of films as separate and distinct production steps, often requiring multiple handling and drying, frequently requiring special procedures to assure adequate bonding between the coating and substrate. It is an object of the invention to provide a simplified method for coating water containing sheets with an aqueous dispersion of polymeric coating material.